The present invention relates to an apparatus for processing a digital signal and more particularly to an apparatus for processing a digital signal employing a method of predictive coding suitable for high quality recording, reproduction, and transmission of such signals as an audio signal.
There has so far been known an apparatus for processing digital signals employing a method of adaptive predictive coding (APC). In a feed-forward predictive coding apparatus, an optimal predictive residual signal, which is a difference between an input signal and an output of an adaptive predictive filter to which the input signal is supplied. On the other hand, in a feedback predictive coding apparatus, while an optimal predictive residual signal being a difference between an input signal and an output of an adaptive predictive filter is obtained, a sum of the optimal predictive residual signal and the output signal of the adaptive predictive filter is supplied to this predictive filter as the input signal thereto.
In either type of these, the adaptive predictive filters are adapted to change their characteristics according to the spectral form of the input signal at a predetermined length of time, and the parameters representative of the filter characteristics are transmitted together with the optimal predictive residual signal or its requantized signal.
There has therefore been a problem that the quantity of the information to be transmitted is increased by the quantity corresponding to the parameters representing the filter characteristics and the efficiency of transmission is lowered accordingly.
To solve this problem, there has been proposed a so-called selective predictive coding apparatus, in which a plurality of predictive filters having predetermined, mutually different, filter characteristics are provided, so that an optimal predictive filter thereof can be selectively used. With this apparatus, a signal for selecting a predictive filter may be transmitted instead of the above described parameters representative of the filter characteristics, and therefore, the efficiency of transmission can be improved accordingly.
However, there has been a problem with the use of the selective predictive filter that the quality of sound of the voice signal obtained by decoding the transmission signal becomes inferior to that obtained with the use of the adaptive predictive filter.